This invention relates to the field of magnetic resonance imaging (MRI) utilizing nuclear magnetic resonance (NMR) phenomena. It is particularly directed toward the shielding of RF coils in an MRI system from extraneous noise sources.
This patent application is related to commonly assigned issued U.S. Pat. No. 4,829,252--Kaufman and to commonly assigned copending U.S. patent application Ser. No. 07/546,112 filed Jul. 2, 1990 naming Kaufman et al as inventors and entitled "MRI Magnet with Robust Laminated Magnetic Circuit Member and Method of Making Same," now abandoned. The entire contents of this related issued patent and of pending U.S. patent application are hereby incorporated by reference into this application.
MRI systems from many different sources are now well-known and commercially available. Magnetic resonance spectroscopic imaging (MRSI) systems are also known and are hereinafter intended to be included within the terminology "MRI" system.
MRI systems typically include a relatively massive static magnet structure for creating a static magnetic field B.sub.o. The static magnet may include a solenoidal cryogenic super-conducting electromagnet or may be of a permanent magnet design. Whatever form of static magnetic is used, it is also typically used in conjunction with a plurality of magnetic gradient coils which are sequentially pulsed to create a sequence of controlled gradients in the static magnetic field during an MRI data gathering sequence. Such controlled sequential gradients are effectuated throughout a patient imaging volume that also is coupled to at least one MRI RF coil. As a part of a typical MRI data gathering sequence, MRI RF signals of suitable frequencies are transmitted into the imaging volume and NMR responsive RF signals are then received from the imaging volume via one or more RF coils or antennae. Information encoded within the frequency and phase parameters of the received RF signals is then processed to form visual images representing the distribution of NMR nuclei within a cross-section or volume of the patient within the imaging volume of the MRI system.
The above-noted related U.S. Pat. No. 4,829,252 describes an MRI system using permanent magnets with open access to the patient image volume and is particularly suited for use in the exemplary embodiment of this invention. To reduce imaging artifacts caused by eddy currents induced in the static magnet structure from changing magnetic flux of the gradient coils, the above-referenced related copending application proposes lamination of the static magnet pole tips. As will be described below, such lamination has been discovered to increase RF noise sources and thus to provide increased need for its shielding from the MRI RF coil(s).
The generation of high quality MRI depends strongly upon the quality of the RF receiving antenna used in the data gathering procedure. There are some RF noise sources inherently present in the process (e.g., within the patient body being imaged) but it is important that care be taken to avoid introduction of any additional unnecessary noise sources into the received RF signals.
Recalling the reciprocity theorem, it will be recognized that one way to assess sources of received RF noise is to measure the power absorbed when the RF antenna is used as a transmitter. Anything which absorbs power in the transmitting mode will also be a source of noise when the same antenna is used as a receiver.
There are three general sources of such power absorption. One is in the antenna itself and that is typically minimized through careful design and use of high quality materials. Another non-avoidable source of noise is in the patient tissues being imaged (albeit surface coils or other techniques can be use for minimized coupling to areas of the patient not actually of image interest). The third category of possible RF power absorption (and therefore a noise source during the RF reception mode) is in the static magnet and/or other structures surrounding the RF coil.
The degree of RF power absorbed in the static magnet or other surrounding structures can be strongly dependent upon the details of magnet design and construction. For example, as above-noted, it has been discovered that adding laminations to the static magnet pole tip (as described in related copending application Ser. No. 07/546,112), now abandoned greatly increases RF power absorption in the static magnet structure.
Generally located closest to the RF receiving antenna is the set of magnetic gradient coils and their associated structure. Although this is a possible source of power loss due to the presence of conducting wires throughout the gradient coil structures, experience has indicated that such losses are typically not an excessive problem.
However, as briefly noted above, experience has now indicated that the static magnet structure itself (typically located outwardly of the gradient coil structures) may be a source of significant power absorption and therefore a source of significant RF noise during the receiving process. For example, when a laminated pole tip structure for the static magnet is used, it has been noted that a receiving coil having a Q of approximately 800 when located outside the magnet may have its Q cut to approximately one-half that value when situated in its operational position between the laminated magnetic pole pieces.
Of course it is well-known in the literature that one can shield an RF antenna from external influences by surrounding the antenna with a good conductor. However, in the context of MRI, such shielding itself produces still further potential imaging artifacts since a conductive surface gives rise to eddy currents generated by changing magnetic flux of the gradient coils (the magnetic field of such eddy currents in turn causing undesirable changes in the magnetic field distribution in the imaging volume). Better conductivity of the shield makes it a better shield--but also causes such eddy currents to take longer to decay to zero and therefore causes greater potential imaging artifact.
The traditional approach to limitation of eddy currents in a conductive member is to create laminations or cuts in the conductor.